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As the Cheshire Cat explains to Alice, the residents of Wonderland do not know how to look back at their past, the reason that they are able to enter Wonderland after all. Hence, what stays "broken" stays broken, and they have no knowledge on how to mend it. Hence, the Duke is a necessary existence that swallows the residents' "regrets" and abandoned pasts. The abandoned story is then ignored and its pages are thrown away. Furthermore, the Duke's role is to quietly collect waste in Wonderland.

Given its obsession with Alice, when it becomes unable to deal with the waste it holds, it begins to want Alice for itself and goes berserk. This is the reason why a "replacement" for Alice such as the Duchess is important, for she distracts the Duke into thinking that she is Alice, thereby protecting the real Alice.

The Duke is a dark, enormous monster-looking creature which resembles an accumulation of black ink. Though it is hardly human in appearance, it is seen to have features such as a mouth, hands, and legs. Several times, it can be seen originating from or taking the form of black oozes which would later concretise into a gigantic creature; it also drips black ink wherever it goes, being called by few people as "filthy". The Duke's size varies greatly across chapters and can be implied to be growing in size the more that it eats away regrets. A single hand can be the size of at least one person, and its body can range to be the size of a house.

The Duke swallows the regrets of Wonderland's inhabitants; as such, it has an important ability to prevent Wonderland from overflowing with regrets. Because it is a collection of regrets, it is advisable to avoid killing it, or Wonderland will flow with regrets. Moreover, being a monster, the Duke's body can be seen to be heavily acidic, or at least has an acidic-like texture, as seen several times when the Duchess comes in physical contact with the Duke, acquiring burn-like wounds on her skin. It can also focus an attack to a certain point where the victim suffers a huge burn on the area affected. In the case of the Cheshire Cat when he protected the Duchess from an attack intended for her, he acquired a huge burn-like injury which affected most of his back.

When the Duchess welcomes the 89th Alice into her residence, the Duke arrives looking for Alice. This triggers the Duchess to let Alice escape to let her perform her duty as Alice's "substitute".[2] Alice sees this creature and tries to kill it, but he is stopped by the Cheshire Cat. When the Duchess welcomes the monster, it drags her away, with the Duchess later seen to have acquired several wounds.[3]

Later, when Alice arrives with the Mad Hatter in Caterpillar Lane, the Duke appears, seemingly having the knowledge of the location of the "real" Alice. The monster causes a ruckus in the town square, possessively looking for Alice. In his castle, the Queen of Hearts asks for a report on what the "substitute" is doing, to which he is told that she is in her mansion. At this point, because of the necessity of the Duke to swallow filthy regrets, the Queen is reminded that Wonderland is a trash can, shocking the Knave of Hearts.

Back at the town square, the Mad Hatter confronts it and, having left with no choice, decides to shoot it. He is suddenly interrupted by the Duchess who says that killing it will result in Wonderland being filled with regrets. She then comes to it and states that "Alice" is now there, but the Duke realises that she is not Alice, angering it and landing an attack on the Duchess, which actually landed on the Cheshire Cat's back after shielding her.[4] The Hatter decides to shoot the Duke, no matter how distant it is from them. The Duchess worries, but the Cheshire Cat reassures that the Hatter is a skilled marksman, with the latter asserting his ability. As he shoots, Alice shouts at him, making him miss his target. The Duke focuses his attention on Alice who proclaims to be the real Alice and eats him up, much to the surprise of everyone at the scene. Inside, Alice marvels at the various objects eaten by the Duke and tries to make a plan to stop him. However, black oozes from the Duke drop at him, injuring him all over.[5] The Hatter tries to shoot it again, but the Duchess stops him, saying that Alice is inside the monster.

Later, the White Rabbit arrives and slashes the Duke with his rapier, retrieving Alice. As the Duke is killed, the White Rabbit makes haste to look for the Duke's replacement to continue cleaning Wonderland free from unnecessary regrets.[6]